Research on what is now called quantum computing was noted by Richard Feynman. See Feynman, 1982, International Journal of Theoretical Physics 21, pp. 467-488, which is hereby incorporated by reference in its entirety. Feynman observed that quantum systems are inherently difficult to simulate with conventional computers but that observing the evolution of an analogous quantum system could provide an exponentially faster way to solve the mathematical model of a system. In particular, solving a model for the behavior of a quantum system commonly involves solving a differential equation related to the Hamiltonian of the quantum system. David Deutsch observed that a quantum system could be used to yield a time savings, later shown to include exponential time savings, in certain computations. If one had a problem, modeled in the form of an equation that represented the Hamiltonian of the quantum system, the behavior of the system could provide information regarding the solutions to the equation. See Deutsch, 1985, Proceedings of the Royal Society of London A 400, pp. 97-117, which is hereby incorporated by reference in its entirety.
One limitation in the quantum computing art is the identification of systems that can support quantum computation. As detailed in the following sections, a qubit, which is analogous to a “bit” of a classical digital computer, serves as the basis for performing quantum computation. However, in order to perform such quantum computations, qubits must be able to retain their quantum behavior long enough to perform quantum computations. The loss of quantum behavior is referred to as decoherence. Further, techniques for reading the state of qubits are needed in order to determine the result of a quantum computation. Ideally, such readout mechanisms do not introduce a source of decoherence to the quantum computing system.
The computing power of quantum devices increases as the basic building blocks of a quantum computer, qubits, are coupled together in such a way that the quantum state of one qubit affects the quantum state of each of the qubits to which it is coupled. There is a form of coupling referred to as entanglement, in qubits are not physically connected to each other yet their states are highly correlated. Another limitation in the quantum computing art is the identification of methods that can be used to controllably entangle the states of qubits without introducing a significant source of decoherence.